In response to shear stress of flowing blood, endothelial cells secrete a number of factors, including nitric oxide (NO), which play a key role in regulating vascular homeostasis. Defects in NO release lead to endothelial dysfunction and contribute to cardiovascular disease, including hypertension, restenosis and atherosclerosis. Our pioneering studies during the previous cycle identified Rap1 as a novel, critical regulator of endothelial cell shear sensing and nitric oxide release. The physiological significance of our finding is underscored by the phenotype of endothelial cell (EC)-specific Rap1 knockout mice, which include endothelial dysfunction and hypertension. Our pilot studies also suggest that Rap1 is required for transmission of shear stress signals from Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1) to Vascular Endothelial Growth Factor 2 (VEGFR2) transactivation and downstream signaling to endothelial NO Synthase (eNOS). The goal of this proposal is to examine the role of Rap1 in transducing shear stress signals required for maintaining EC homoeostasis. The overall hypothesis is that Rap1 promotes shear stress-induced signals from the mechanosensing receptor PECAM-1, via its effector, Afadin, to VEGFR2 and downstream to eNOS. Further, disruption of Rap1 function promotes pro-inflammatory endothelial phenotype and exacerbates atherosclerosis. The hypothesis will be tested in three aims. Aim 1 will examine molecular mechanisms of Rap1 activation in response to shear stress. Studies utilizing immortalized human endothelial cells expressing PECAM-1 mutants will determine the involvement of Rap1 activator (Rap1 GEF) C3G and Rap1 effector, Afadin, in transmission of signals from PECAM-1 to VEGFR2 activation and downstream signaling. Aim 2 will investigate the role of Rap1 in transducing laminar and disturbed flow signals. Utilizing Rap1-deficient ECs in vitro and vessels from endothelial-specific Rap1 knockout mice ex vivo, the effect of shear on acute signaling and long-term pro-inflammatory gene expression will be examined. Aim 3 will examine the effect of disrupted Rap1 signaling as a factor exacerbating endothelial function leading to a pro-inflammatory state in vivo. The studies will investigate the effect of endothelial Rap1 deletion on progression of atherosclerosis in a mouse model in vivo. Proposed studies will uncover novel, previously unexpected mechanisms governing EC responses to shear and may lead to a new direction in restoring EC function by controlling Rap1 signaling.